This invention relates to a novel method for removing insolubilized PVA (polyvinyl alcohol-acetate) portions from the surface of a body and particularly, but not exclusively, to a method for stripping an etch-resistant PVA stencil from a metal body after the body has been etched.
The preparation of apertured masks for use in cathode-ray tubes by photoexposure and etching has been described previously, for example, in U.S. Pat. No. 4,061,529 issued Dec. 6, 1977 to A. Goldman et al. In a typical process, light-sensitive coatings of sensitized natural protein materials are applied to both major surfaces of a thin metal sheet, such as a sheet of a cold-rolled steel or of a copper-nickel alloy. The coatings are exposed to light images, as by contact printing exposure, to harden (render less soluble) the exposed portions of the coatings in an aqueous solvent. The exposed coatings are developed by removing only the more-soluble portions of the coatings, thereby producing a stencil on each surface of the sheet. Then, the stencils are baked to make them more resistant to an etchant for the metal sheet. Usually the etchant is an aqueous solution of ferric chloride and hydrochloric acid. Then, the sheets with the stencils thereon are selectively etched as desired, after which the stencils are removed from the sheet. In present commercial practice, the light-sensitive coatings are dichromate-sensitized casein; such as the coatings disclosed in the above-cited Goldman et al. patent. Etch resistance is developed by baking the coatings in air at about 200.degree. C. to 350.degree. C. After etching the metal sheet with a ferric chloride-hydrochloric acid solution, the stencils are removed by applying thereto a hot aqueous solution of sodium hydroxide or other alkali.
It is desirable for several reasons to provide alternative light-sensitive coatings, particularly light-sensitive synthetic materials, in place of the light-sensitized natural materials that are presently used. The use of acid-resistant stencils of insolubilized dichromate-sensitized PVA has been suggested in the prior art. However, the prior art has recognized that it is difficult to completely remove the stencils in an industrial and cost-effective manner with alkali solutions. U.S. Pat. No. 4,208,242 issued June 17, 1980 to P. Zampiello suggests using films of dichromate-sensitized polyvinyl alcohol-acetate as the light-sensitized coating for making apertured masks in steel sheets. The PVA employed in this film is 98.5% to 100% hydrolyzed and has a molecular weight of about 14,000. The film, after developing, is baked at about 250.degree. C. for about 2 to 3 minutes to improve the acid resistance thereof. This produces very adherent stencils which require a special treatment to remove them. That treatment requires, after etching the sheet, baking the stencils at about 250.degree. to 300.degree. C. until the stencils are carmelized. Then, the carmelized stencils can be removed with hot aqueous alkali sodium hydroxide solutions.
U.S. Pat. No. 4,339,528 to A. Goldman suggests avoiding the post-etch baking step employed in the Zampiello method by using a PVA selected from a small group defined by degree of hydrolysis and molecular weight. U.S. Pat. No. 4,339,529 to A. Goldman suggests avoiding the post-etch baking employed in the Zampiello process by adding NMA (N-methylol acrylamide) to the resist composition. In both of these patents, the acid-resistant stencils are stripped off with a hot aqueous alkali solution.
In other applications which use insolubilized PVA stencils for etching, the stencil is removed using chemicals that are chemically oxidizing to insolubilized PVA chemicals. As examples, U.S. Pat. No. 3,175,907 to A. Fishman discloses treating the stencil with a hot aqueous solution of a peroxide or a perborate; U.S. Pat. No. 3,415,648 to A. J. Certa discloses treating the stencil with hot aqueous chromic acid; and U.S. Pat. No. 3,796,603 to Cox et al. discloses treating the stencil with an aqueous solution of a permanganate or a periodate. In the Cox et al. patent, the periodate may be periodic acid.
Each of these prior methods for removing insolubilized PVA portions from the surface of a body has at least one of the following disadvantages: (1) it is too slow, (2) PVA removal is incomplete, (3) it is too costly and/or (4) it is not well adapted to be implemented by machine. The novel method is relatively fast, the removal is complete, it is cost effective and it can be implemented by machine.